For a law to be considered constitutional under the Establishment Clause of the First Amendment, the law must have a legitimate secular purpose, must not have the primary effect of either advancing or inhibiting religion, and also must not result in an excessive entanglement of government and religion.

Lemon v. Kurtzman, 403 U.S. 602 (1971),[1] was a case argued before the Supreme Court of the United States. The court ruled unanimously in an 8-0 decision that Pennsylvania's Nonpublic Elementary and Secondary Education Act (represented through David Kurtzman) from 1968 transgressed the Establishment Clause of the First Amendment. The act allowed the Superintendent of Public Schools to reimburse private schools (mostly Catholic) for the salaries of teachers who taught in these private schools, from public textbooks and with public instructional materials. The decision also upheld a decision of the First Circuit, which had struck down the Rhode Island Salary Supplement Act providing state funds to supplement salaries at private elementary schools by 15%. As in Pennsylvania, most of these funds were spent on Catholic schools.

The act stipulated that "eligible teachers must teach only courses offered in the public schools, using only materials used in the public schools, and must agree not to teach courses in religion." Still, a three-judge panel found 25% of the State's elementary students attended private schools, about 95% of those attended Roman Catholic schools, and the sole beneficiaries under the act were 250 teachers at Roman Catholic schools.

The court found that the parochial school system was "an integral part of the religious mission of the Catholic Church," and held that the Act fostered "excessive entanglement" between government and religion, thus violating the Establishment Clause.[1]

Held: Both statutes are unconstitutional under the Religion Clauses of the First Amendment, as the cumulative impact of the entire relationship arising under the statutes involves excessive entanglement between government and religion.[1]